1. Field of the Invention
The present invention relates to a charge completion detecting device which outputs a charge completion signal when a main capacitor for energizing a flash tube is charged to a given level, and especially to such a charge completion detecting device for a flash photography system wherein signals are exchanged between a camera circuit and an electronic flash device circuit for their mutual control.
2. Description of the Prior Art
Recently, a flash photography system has been proposed wherein the amount of flash light emitted from an electronic flash device to an object to be photographed is controlled in accordance with a signal fed from a camera. A camera of such a system obtains a distance signal representing the distance from the camera to the object by means of the adjusted position of the focus adjusting ring provided on a lens barrel. Another camera obtains the distance signal from an automatic focus detecting device provided in the camera. Those cameras calculate a datum of the appropriate or required amount of flash light, from the object distance signal obtained as explained above, along with data of the manually set film sensitivity and a manually preset or object light dependently automatically determined diaphragm aperture value. The cameras then feeds the datum of the required flash light amount to an electronic flash device, which compares the datum from the camera with a signal corresponding to the amount of flash light measured directly from a flash tube and integrated, and controls the amount of flash light emitted from the flash tube in accordance with the comparison.
In another flash photography system, the exposure control mode of a camera is automatically switched from a day light photography mode to a flash light photography mode in response to a charge completion signal fed from an electronic flash device when the main capacitor of the device is charged to a given level. With the mode switching, the shutter speed to be controlled is changed over from a proper value determined in accordance with a light measurement to a flash synchronizable value suitable for flash photography.
In such flash photography systems, the voltage level which the main capacitor is to attain for the charge completion is fixed to a value, e.g. to 300 V corresponding to a substantially fully charged condition for presenting the maximum amount of flash light. In other words, the time interval from the start of charging to its completion is always constant in so far as the initial voltage of the main capacitor is the same. The charging state of the main capacitor is treated as uncompleted until the charged voltage reaches the given full voltage, although in photographic conditions where the object is close to the camera or the diaphragm aperture is opened widely, the electronic flash device can emit a sufficient amount of light to the object even if the main capacitor has not been fully charged. If shutter release operation is made under such uncompleted conditions, a proper flash photography with a proper amount of flash light in accordance with the signal from the camera is not effected, e.g. the electronic flash device is not fired or the camera exposure control mode is not changed to the flash light photography mode, and a photographic opportunity may be lost.
An independent electronic flash device has been proposed which can change the reference voltage for the detection of charge completion in accordance with the manual operation for adjusting the amount of flash light to be emitted from the electronic flash device. But this reference voltage changing operation is made independently of the photographic conditions of the camera, so that the prior art device requires manual changing or adjusting operation for both the flash device and the camera resulting in inconvenience and complexity of handling.